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www.fugro.com
Ocean Bottom Node Acquisition
8 March 2012
Bjorn Olofsson (SeaBird Exploration, on behalf of Fugro)
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Ocean Bottom Node Acquisition – What is it?
4 component seismic sensor: 3 geophones (XYZ) - also MEMS or optical for OBC 1 hydrophone
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Outline
• OBN Acquisition – Why is it done?
• Equipment and node operation
• Full azimuth survey
• Data processing
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OBN Acquisition
Why is it done?
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OBN Acquisition – Why is it done? Complex imaging with full
azimuth broad band data
Beaudoin, SEG 2010 Atlantis field, GoM
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OBN Acquisition – Why is it done? High resolution both vertically
and laterally
Howie et al, SEG 2008 Atlantis field, GoM
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OBN Acquisition – Why is it done? 4D Repeatability
“…high repeatability with a NRMS of around 6%.“ Stopin et al, EAGE 2011
Base line survey 4D difference
“Time-lapse noise measurements [...] are among
the lowest in BP’s experience even when
compared to permanent installation surveys.”
Atlantis field, GoM Reasnor et al, SEG 2010
Mars field, GoM
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OBN Acquisition – Why is it done? Infill under obstructions,
congested oilfields
Dalia field, Angola
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OBN Acquisition – Why is it done? Infill under obstructions,
congested oilfields
FPSO (=Floating Production, Storage and Offloading unit)
Offloading buoy
Drilling ship ..another FPSO
?
Supply boat Hugin Explorer
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OBN Acquisition – Why is it done? Infill under obstructions,
congested oilfields
110°F
Source array firing
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OBN Acquisition – Why is it done? Deep water,
Unconventional methods
• Nodes can be hand-placed almost anywhere
• Passive recording, long offsets...
• Ocean bottom nodes can carry many types of sensors
OBN unit
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OBN Acquisition – Why is it done?
Beaudoin, SEG 2010
PP PS
Fjellanger et al, SEG 2006 Alba field, North Sea
Lomond field, North Sea
Grane field, North Sea
Converted wave
imaging Shown are classic OBC examples
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Why Converted Waves? PP & PS = Improved reservoir
characterisation
PP AVO inversion P impedance
PS AVO inversion Shear impedance
..also better handle on density. PP
PS
Ao & Areklett, TLE 2010
Kvitebjorn field, North Sea
Strong shear impedance contrast from lithology change within reservoir zone.
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Why Converted Waves? PP & PS = Better anisotropic velocity
model building
PP reflection, isotropic NMO correction
PS reflection, isotropic NMO correction
Double Scan
PP
PS
PP & PS
Epsi
lon
Delta
Note polarity reversal at critical angle
Offset/Angle of incidence
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OBN Acquisition
Equipment and node operation
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OBN Equipment – Nodes
Option 1: Throw node overboard, let it float up by itself
• Typically glass or titanium sphere • Disposable heavy anchor • Internal or external sensor package • Mostly used for academic research or refraction lines EM node
…illustration of node equipment from various vendors (& academia)
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OBN Equipment – Nodes
• Node can be custom shaped • Recorder in cylindrical pressure vessels • Internal or external sensor package • Mostly used for commercial 3D surveys
…illustration of node equipment from various vendors
Option 2: Hand-place node, pick it up manually
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Node and sensors
3 geophones (8Hz)
1 hydrophone (1.5Hz analogue low cut)
2 inclinometers
Low frequency gain compared to other systems:
Hugin Explorer – Node/Source/ROV vessel
ROV
Node deck Dual source capability
Second ROV
Node storage capacity: 1200+
• Usually operates in dual-boat mode with a separate source
vessel
• In one survey the Hugin operated alone, deploying/retrieving
nodes AND shooting while towing a 600m mini-streamer
OBN Technology – Vessel and equipment
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OBN Equipment – Node Handling
ROV Node basket
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OBN Operation – Node Placement
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OBN Operation – Node Placement
“Added mass”
contribution
from soil
Sensor skirt
(cutaway view)
Unperturbed
soil
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OBN Operation – Node QC
Acoustic modem communication
between OBN and vessel
• Recorder status • Battery status • Hard disk status • Power usage • Tilt values • Seismic data RMS
amplitudes • ...various other system
information
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OBN Acquisition
Full azimuth survey
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Node area
Shot area
Node area
Shot area
Basic node survey design consists of regular node grid and encompassing shot grid. Illustration of contributing receivers/shots for two example bins:
Contributing node locations
Contributing shot locations
Bin
Bin
Centre bin Corner bin
OBN Acquisition – Full azimuth survey
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Rose diagram – Offset/azimuth fold:
(similar OBN survey)
Offset fold diagram for the 2 example bins:
Poor near offset fold??
OBN Acquisition – Full azimuth survey
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Corner bin Centre bin
OBN offset/azimuth distribution is best viewed in common-offset vector tiles (OVTs). For every CMP bin, contributing shot-receiver pairs are evenly distributed on a regular inline/crossline offset grid. Pre-stack migration is best performed in OVTs.
OBN Acquisition – Full azimuth survey
Even offset distribution in every azimuth direction
Within the limits of survey area, every CMP bin has one trace in each offset vector tile.
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Corner bin Centre bin
OBN Acquisition – Full azimuth survey
Size of OVT is governed by node spacing
Size of CMP bin is governed by shot spacing
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OBN Acquisition
Data Processing
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Continuous recorded data
Active shooting DC shift
• Active shots need to be extracted from continuous record, using shot time
• Shot time needs to be mapped to time of internal clock
• Clocks used in OBNs are very accurate, but can still drift by several 10ms per month
• New clocks only drift ~0.1ms per month
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OBN Data processing – Spectral analysis
Electrical
“1/f” noise
Ocean
wave noise
Decay due to
sensor responses
and diminishing
shot energy
Active shot energy.
Ripples due to bubble
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Raypath geometry for one node/receiver gather:
Pre-processing is done mostly in 3D receiver gather domain.
OBN Data processing
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X Y Z Hydrophone
Example raw receiver gather, deep water (~1km)
First water
bottom multiple
Direct arrival
Second?
Shear noise
Zero offset P-wave
reflection
PS
converted
waves
Bubble
OBN Data processing – Raw data example
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OBN Data processing – Multiple path imaging
Conventional imaging Primary reflections, up-going wavefield
Multiple path imaging Receiver side multiple, down-going wavefield
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Pre-conditioning
Noise attenuation
PZ calibration
Vz noise removal
Source designature
Wavefield separation
Up/down decon
TTI PSDM
Radon demultiple
Mute & Stack
Post-stack processing
Essential data processing steps (fast-track)
Acquisition-related corrections:
Positions, clock drift, tides…
Seismic interference, ground roll
(shallow water)
At least: Remove bubble oscillations
Shear induced noise on Z component
Apply to upgoing energy
Separate into up- and downgoing
energy, process separately
Kirchhoff migration, using existing
velocity field. Normal/multiple path
Q compensation
OBN Data processing – P-wave imaging
Match hydrophone and geophone
Surface consistent processing
Timing corrections
3D SRME
Offset vector tiles
Interpolation / regularisation ?
Anisotropic velocity model building
RTM / Kirchhoff ?
Azimuthal velocity correction
Advanced data processing steps
Surface consistent scaling,
deconvolution, statics
Advanced tidal and water
velocity corrections
Apply to downgoing energy
• Use 3D methods (noise attenuation..)
• Integration of well data
• Attention to details
Binning
..more generally:
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OBN Data processing – Multiple path imaging
Conventional imaging Primary reflections, up-going wavefield
Multiple path imaging Receiver side multiple, down-going wavefield
…from Dash et al (2009)
Same aperture
Wider aperture
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OBN Data processing – Multiple path imaging
Upgoing migration Downgoing “mirror” migration
Illustration from single 16-node line mini 2.5D survey (essentially 2D geometry)
processed by Geokinetics
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X Z P X Z P
X Z P
1Hz - 2Hz
4Hz - 5Hz
2Hz - 3Hz Raw data, band-pass
filtered, T1 gain
X Z P
3Hz - 4Hz
± 25km
OBN Data processing – Low frequency content
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1Hz - 2Hz
4Hz - 5Hz
2Hz - 3Hz
3Hz - 4Hz
1Hz - 20Hz
..different scaling
OBN Data processing – Low frequency content
Basalt layer
Fully migrated 2D section, band-pass filtered
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OBN Acquisition – Summary
Upsides • Operationally efficient in presence of…
‒ Surface obstructions (impeding use of towed streamer) ‒ Seabed obstructions, rugged seafloor (impeding use of ocean bottom cables/OBC)
• Full azimuth: Full & even surface azimuth/offset distribution • Vector fidelity: Ideal sensor coupling, decoupled sensor, no distortion due to instrument • High resolution: Naturally rich in low frequencies, no compromise at high end • Low ambient noise: Swell/flow noise (streamer), water current noise (OBC) • 4D repeatability: Repeatability of positions, and of final processed data (low NRMS) • Full elastic wavefield recording: 4 component sensor, P- and S-wave arrivals • Continuous recording: Passive seismic monitoring & analysis Downsides • Autonomous recording
‒ Requires high fidelity clock drift correction ‒ Node reliability is crucial
• Sparse receivers ‒ Resolved by multiple (mirror) imaging, but problematic for converted waves
• Survey time & cost ‒ Needs good evaluation of added value
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Thank You
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Spectral analysis
Shot lines
Shot fired
Recorder noise
Ocean wave noise
ROV placing node at 5m distance
1
2
3 4
5
6
7
8
Ship
Continuous data spectra – 4 minute traces X Component
Seismic interference
Test shots
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Spectral analysis
Test shots
Seismic interference Shot fired
Recorder noise
Shot lines
Ocean wave noise
ROV placing node at 5m distance
Ship
Continuous data spectra – 4 minute traces Y Component
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Spectral analysis
Test shots
Seismic interference Shot fired
Recorder noise
ROV hoisted on deck
Shot lines
Ocean wave noise
ROV placing node at 5m distance
Ship
Continuous data spectra – 4 minute traces Z Component
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Spectral analysis
Test shots
Seismic interference Shot fired
Recorder noise
ROV hoisted on deck
Ocean wave noise
Shot lines
Ship
Continuous data spectra – 4 minute traces Hydrophone
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Spectral analysis
5 hours of recording 5 hours of recording
Earthquake/ Seaslide
Same spectrum, zoomed in to 0-0.7Hz
Continuous data spectra – 4 minute traces Hydrophone
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Boundary of shot grid (surface) 588sqkm
Boundary of node grid
(ocean bottom) 229 sqkm
O/W contact
OBN Survey – Node and source area
Another example:
Node/shot area is optimised
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• 1595 total node positions • Node grid: 390m x 390m
OBN Survey – Node layout
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• 648,648 total shot positions • Shot grid: 30m x 30m • Shooting vessel acquiring
one shot line at a time
Example sail line for shooting vessel
OBN Survey – Source layout
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First node line
N
13-line shot swath
OBN Survey – Roll-along acquisition
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OBN Survey – Roll-along acquisition
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OBN Survey – Roll-along acquisition
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OBN Survey – Roll-along acquisition
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OBN Survey – Roll-along acquisition
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OBN Survey – Roll-along acquisition
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25th shot swath
OBN Survey – Roll-along acquisition
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OBN Survey – Roll-along acquisition
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OBN Survey – Roll-along acquisition